The cathodes of DC gas discharge lamps (e.g. mercury discharge lamps, xenon discharge lamps) are generally doped in order to reduce the work function thereof and thus to obtain a lower operating temperature of the cathode. For this purpose, as standard, use is made of ThO2 as emitter material, which is distinguished by a particularly high vaporization temperature. Substitutes (e.g. oxides of lanthanides and/or further oxides, e.g. ZrO2, HfO2) can now reduce the work function of W to a comparable extent; see e.g. Manabu Tanaka et al. 2005 J. Phys. D: Appl. Phys. 38 29 (2005). Accordingly, tip temperatures of approximately 3400 K can be achieved e.g. by addition of 2% by weight La2O3. In the context of said study, the values—shown in the table below—of the temperature of the tip of a tungsten electrode during the operation of a freely burning argon arc were determined experimentally using a pyrometry method with a continuous wavelength. In this case, the arc length was 5 mm, the protective gas was argon, and the cone angle of the cathode was 60°. The electrode material is indicated in the first column, and the temperature of the cathode tip in kelvins is respectively indicated in the second and third columns for a cathode current of 100 A and for a cathode current of 200 A.
TemperatureTemperatureElectrode material[K] (100 A)[K] (200 A)Pure tungsten40624560W-2% ThO236953723W-2% La2O333523481
However, all the substitutes have a melting point that is hundreds of kelvins lower, such that the emitter evaporates to a much greater extent during operation. This is illustrated in the table below, which lists the melting points of some oxides. The enthalpies of vaporization and the melting points of the associated elements are also generally lower than in the case of thorium, but only a very small part of the emitter is present in elemental form, for which reason the melting points of the compounds are more meaningful.
Enthalpy ofvaporizationMelting pointBoiling point[kJ mol−1][K][K]Cerium31910683716Ce2O32503Hafnium66125064876HfO23047Lanthanum40011933737La2O32578Neodymium28412973373Nd2O32593Samarium19213452076Sm2O32608Scandium30518143103Sc2O32758Thorium54421155061ThO23363Zirconium58221284682ZrO22950Tungsten4233680
The fact that emitters not containing thorium evaporate more readily leads, inter alia, to severe bulb blackening and a shorter lamp lifetime. Owing to this poorer performance, cathodes including Th substitutes have not yet been able to gain acceptance, even though they would be preferable for environmental protection reasons.
Unthoriated cathodes have not yet gained acceptance in the lamp sector. Although alternatives to thorium oxide are described in many instances in the patent literature (e.g. addition of oxides of La, Nd, Sm, Zr), three different problems occur in the case of these cathodes.
(1) The emitter transport to the tip is generally not constant owing to the higher emitter evaporation at the tip. The following process takes place (periodically): emitter evaporates at the cathode tip, owing to the lower vaporization temperature of thorium substitutes. The tip temperature rises as a result of the emitter depletion. It is finally so high that emitter from the volume material, the so-called bulk, is transported to the tip again. The temperature falls and the subsequent transport comes to a standstill. The emitter vaporizes, the tip is depleted, the temperature rises, etc. As a result of this process, the work function at the tip is permanently altered, and lamp flicker occurs. This flicker is manifested in both voltage and intensity changes as a result of e.g. arc contraction or altered arc attachment regions. As a result, the cathode becomes unusable for most applications (semiconductor exposure, cinema).
(2) As a result of the flicker and/or as a result of a generally lower deformation temperature, enlargement of the tip and thus loss of intensity occur.
(3) If lamps are actually successfully operated without flicker, the emitter subsequent transport is generally so rapid that the lamps undergo extreme blackening. Use of said lamps is not expedient owing to the thus greatly shortened lifetime.
In this context, EP 1 481 418 B8 discloses a DC gas discharge lamp having a discharge vessel having two necks fitted diametrically oppositely, into which an anode and a cathode each composed of tungsten are fused in a gas-tight fashion, said discharge vessel having a filling composed of at least one noble gas and possibly mercury. At least the material of the cathode tip contains, in addition to the tungsten, lanthanum oxide La2O3, and at least one further oxide from the group hafnium oxide HfO2 and zirconium oxide ZrO2.
WO 2014/038423 A1 discloses a short-arc discharge lamp containing a rare earth oxide as an emitter substance in a cathode of a fluorescent tube in which a structure is provided in which the rare earth oxide as emitter substance can be protected from evaporating excessively from the cathode and its premature exhaustion can therefore be prevented. A cathode includes a cathode body and a cathode tip connected to the tip of the cathode body, wherein the cathode body includes tungsten which contains a rare earth oxide as emitter substance, and the cathode tip comprises tungsten which does not contain any emitter substance.
WO 2013/113049 A1 describes an electrode of a high-pressure gas discharge lamp which includes a core composed of tungsten or tungsten doped with potassium having a diameter di and a shell adjacent thereto having an external diameter da, wherein the shell, at least in some regions, consists of a particle composite material including a matrix of tungsten and the following condition is met: di≤da/3. The electrode described therein is said to be distinguished by a significantly reduced arc instability.
Hitherto it has not been possible to show how all three problems, namely flicker (arc instability), electrode burnback or deformation and blackening of the lamp bulb, can be solved together.